Synthesis of lithium iron sulphides and their use as cathodes

ABSTRACT

A process for the production of a lithium transition metal sulphide such as lithium iron sulphide, the process comprising reacting a transition metal sulphide with lithium sulphide in a solvent comprising a molten salt or a mixture of molten salts. Lithium transition metal sulphides obtained using this process are useful in the production of electrodes, in particular for rechargeable lithium batteries.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US national phase of international applicationPCT/GB01/05209 filed 27 Nov. 2001, which designated the US.

The present invention relates to processes for the production ofsulphides, in particular lithium transition metal sulphides useful inthe production of batteries.

BACKGROUND OF THE INVENTION

In the 1980's, there was extensive research into lithium metalrechargeable batteries, particularly using sulphides, but alsoselenides, as cathode materials. Many lithium metal/molybdenumdisulphide (Li/MoS₂) batteries were produced but these were withdrawnfollowing an incident in which a fire was attributed to the malfunctionof such a battery. Other sulphides, such as iron disulphide FeS₂,titanium disulphide TiS₂ and selenides, such as niobium triselenideNbSe₃ have also been particularly investigated as alternative cathodematerials.

Although the use of lithium metal rechargeable batteries is now limitedfor reasons of safety, they are still used in the laboratory testing ofmaterials. Lithium metal primary batteries using iron disulphidecathodes are manufactured.

Virtually all modern lithium rechargeable batteries are of thelithium-ion type, in which the negative electrode (anode) compriseslithium absorbed into a carbon support. These use a lithium containingcathode material, which is usually lithium cobalt oxide LiCoO₂ althoughlithium nickel oxide LiNiO₂, lithium manganese oxide LiMn₂O₄ and mixedoxides are also known to have been used.

Due to their high cost, the use of lithium rechargeable batteries atpresent is mainly limited to premium applications, such as portablecomputers or telephones. To gain access to wider markets, for example inapplications such as the powering of electric vehicles, the cost must bereduced. Hence there is a strong demand for the high performanceobtainable from lithium-ion batteries at much more economical prices.

On first inspection, the use of sulphides as cathode materials is not asattractive as the use of oxides. This is because the voltage achievablefrom sulphides is generally only about half of that achievable usingcorresponding oxides. However, the capacity of batteries incorporatingsulphide based cathodes, measured in ampere hours per gram of material,is about 3 times greater than corresponding batteries incorporatingoxide based cathodes. This leads to an overall advantage of about 1.5times in terms of cathode energy density for batteries with sulphidebased cathodes. A further advantage is that iron sulphides, inparticular ferrous sulphide (FeS) and iron disulphide (FeS₂) areinexpensive materials which may be dug out of the ground as naturaloccurring minerals. By contrast, lithium cobalt oxide is an expensivematerial, due mainly to the high cost of cobalt metal.

Binary transition metal sulphides are however not suitable for directuse in lithium-ion cells as they do not contain lithium. Lithiumtransition metal ternary sulphides, such as lithium molybdenum sulphide,lithium titanium sulphide, lithium niobium sulphide and lithium ironsulphide have been suggested as electrode materials for batteries (seefor example, Japanese Kokai No 10208782 and Solid State lonics 117(1999) 273–276). The conventional synthesis of lithium iron sulphide isvia a solid state reaction in which lithium sulphide, Li₂S, and ferroussulphide, FeS, are intimately mixed together and heated under an inertatmosphere at a temperature of ca. 800° C. The reaction is diffusioncontrolled and the kinetics are slow. Consequently, the reaction cantake up to 1 month at temperature to reach completion. This is highlyinconvenient and is costly in terms of energy input. The economics ofthis synthesis for battery production are clearly unfavourable.

On a laboratory scale, lithium iron sulphide can be made by anelectrochemical synthesis route in which a lithium metal/iron disulphidecell is discharged, and the lithium metal is removed and replaced by acarbon anode. This process however, is not amenable to scaling up. Afurther laboratory synthesis of lithium iron sulphide is the solid statereaction of lithium nitride, Li₃N, with iron disulphide, FeS₂, butagain, this method is unsuitable for large scale use because of the highcost and shock sensitivity of lithium nitride.

The applicants have developed an economical synthesis which can beoperated on a large scale to produce sulphides which have usefulelectrochemical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be particularly described by way of example onlywith reference to the following drawings in which;

FIG. 1 shows an x-ray diffraction trace for the product obtained using afirst example of a process according to the present invention;

FIG. 2 shows cycling curves for the product obtained using a firstexample of a process according to the present invention;

FIG. 3 shows an x-ray diffraction trace for the product obtained using asecond example of a process according to the present invention;

FIG. 4 shows an x-ray diffraction trace for the product obtained using athird example of a process according to the present invention;

FIG. 5 shows cycling curves for the product obtained using a thirdexample of a process according to the present invention; and,

FIG. 6 shows an x-ray diffraction trace for the product obtained using afourth example of a process according to the present invention.

DESCRIPTION OF THE INVENTION

In accordance with the present invention a process for the production ofa lithium transition metal sulphide comprises reacting a transitionmetal sulphide with lithium sulphide in a solvent comprising a moltensalt or a mixture of molten salts.

Suitably the transition metal sulphide used in the process is an iron,molybdenum, niobium or titanium sulphide and is preferably an ironsulphide. Ferrous sulphide, FeS, and iron disulphide, FeS₂, areinexpensive and readily available naturally occurring minerals.

Preferably the molten salt or mixture of molten salts comprises analkali metal halide or a mixture of alkali metal halides, or an alkalineearth metal halide or a mixture of alkaline earth metal halides, or anymixture thereof. More preferably, the molten salt or mixture of moltensalts comprises a lithium halide or a mixture of lithium halides.

Most preferably, the molten salt or mixture of molten salts comprises atleast one of lithium fluoride, lithium chloride, lithium bromide orlithium iodide.

The reaction temperature should be sufficient to liquefy the molten saltor mixture of molten salts. This need not necessarily be the meltingpoint of the molten salt or mixture of molten salts as the addition ofthe reactants may depress the melting point. Typically, reactiontemperatures of less than 1000° C. and most often less than 700° C. aresuitable, however dependent on the choice of solvent, reactiontemperatures of less than 300° C. may be used.

The reaction proceeds more rapidly than previously known processes. On alaboratory scale, the reaction can be completed in a few hours, with theactual reaction time dependent largely on the heating time of thefurnace.

Although lithium sulphide may be bought commercially, for large scaleproduction it is more economical to produce lithium sulphide via thereduction of lithium sulphate. One convenient method is to heat lithiumsulphate above its melting point of 860° C. in the presence of carbon.Other standard reduction methods may equally be used, as well known inthe art.

After the reaction is complete and allowed to cool, the product must berecovered from the solvent. Suitably the product is recovered bydissolution of the solvent in an organic liquid. The organic liquidchosen is dependent on the composition of the solvent used, however someexamples include, pyridine, ether and acetonitrile which are suitablefor the dissolution of lithium chloride, lithium bromide and lithiumiodide respectively. Numerous other suitable liquids will be known tothose skilled in the art. When a mixed salt solvent is used it may benecessary to perform more than one dissolution process. For example, areaction using a mixture of lithium chloride and lithium bromide as asolvent may require a first dissolution process using pyridine to removethe lithium chloride, followed by a second dissolution process usingether to remove the lithium bromide.

The present invention further provides a process for producing at leastone lithium transition metal sulphide by reacting a transition metalsulphide with lithium sulphide in the presence of a molten salt ormixture of molten salts. A plurality of lithium transition metalsulphides may be made by such a process and subsequently separated.

In a further aspect, there is provided a process for producing one ormore lithium transition metal sulphides by reacting one or moretransition metal sulphides with lithium sulphide in the presence of afurther salt with which they do not react, in an inert atmosphere,wherein the salt and at least one component are in a molten state toallow intimate mixing, the salt preferably acting as a solvent for saidat least one component.

Lithium transition metal sulphides obtained by the above describedprocess form a further aspect of the invention. These compounds areuseful in the production of electrodes for use in batteries. Inparticular, they are useful in the production of electrodes forrechargeable batteries. These electrodes form the cathode, and suitableanodes are lithium ion anodes as are known in the art. Suitableelectrolytes are also well known and include mixtures of inorganiccarbonates, for example ethylene carbonate, propylene carbonate, diethylor dimethyl carbonates, ethyl methyl carbonate together with a lithiumsalt, usually lithium hexafluorophosphate, LiPF₆, or lithiumtrifluoromethane sulphonate (‘triflates’), LiCF₃SO₃ or lithiumtetrafluoroborate, LiBF₄.

Molten salts and mixtures of molten salts are not conventional solventsand their use, acting like solvents in the production of sulphides,therefore forms a further aspect of the invention. As described above,they are particularly suitable for use as solvents in reactions used inthe production of lithium transition metal sulphides.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Lithium iron sulphide, Li₂FeS₂ was synthesised according to thefollowing equation:Li₂S+FeS→Li₂FeS₂

Stoichiometric amounts of lithium sulphide, Li₂S, and iron sulphide,FeS, were intimately mixed with a roughly equivalent weight of a salt ormixture of salts which constituted the solvent. The resulting mixturewas placed into a nickel crucible and heated under an inert atmosphereto effect reaction. After the reaction was complete, the crucible andits contents were allowed to cool while still under an inert atmosphere,before being transferred to an inert atmosphere glove box. The salt ormixture of salts was then removed from the desired product by refluxingthe powdered contents of the crucible with an organic liquid. Afterfiltering and drying, the resultant product was analysed by x-ray powderdiffraction (XRD) using a Philips PW1830 Diffractometer and CuKαradiation.

Cell cycling tests were carried out on the product as follows. Cathodesheets were made by the doctor blade method. The product was mixed withgraphite and a solution of ethylene propylene diene monomer (EPDM) incyclohexane to form a slurry. This was then coated onto an aluminiumbacking sheet. Negative electrodes were made by a similar method exceptthat the active material was carbon in the form of graphite with somecarbon black added, the binder was polyvinylidene fluoride dissolved inN-methylpyrrolidinone (NMP) and the metallic backing sheet was copper.The electrolyte was ethylene carbonate (EC)/diethyl carbonate (DEC)/1molar lithium hexafluorophosphate (LiPF₆). Cells were cycled at roomtemperature. This cell cycling procedure is described in more detail byA. Gilmour, C. O. Giwa, J. C. Lee and A. G. Ritchie, in the Journal ofPower Sources, volume 65, pages 219–224.

EXAMPLE 1

Li₂S and FeS were reacted together in a molten salt solvent of lithiumchloride, LiCl, at 650° C. for ca. 2 hours, under an argon atmosphere.After completion, the LiCl was removed by refluxing in pyridine for 8hours. FIG. 1 shows an XRD trace of the product obtained. The verticallines 1 represent the standard trace for pure Li₂FeS₂ taken from theJCPDS database. The main peaks are co-incident with and have similarrelative intensities to these lines 1, indicating that the dominantproduct phase obtained was Li₂FeS₂. The remaining peaks correspond tosmall amounts of unreacted starting materials.

The product obtained was used to manufacture a cathode as describedabove. FIG. 2 illustrates three cycling curves which indicate that thecathode could be repeatedly charged and discharged. This demonstratesthat the product was suitable for use as a cathode material for alithium rechargeable battery.

EXAMPLE 2

Li₂S and FeS were reacted together in a molten salt solvent of lithiumbromide, LiBr, at 550° C. for ca. 2 hours, under an argon atmosphere.After completion, the LiBr was removed by refluxing in diethyl ether for8 hours. FIG. 3 shows an XRD trace of the product obtained. The verticallines 1 represent the standard trace for pure Li₂FeS₂ taken from theJCPDS database. The main peaks are co-incident with and have similarrelative intensities to these lines 1, indicating that the dominantproduct phase obtained was Li₂FeS₂. The remaining peaks correspond tosmall amounts of unreacted starting materials.

EXAMPLE 3

Li₂S and FeS were reacted together in a molten salt solvent of lithiumiodide, Lil, at 450° C. for ca. 2 hours, under an argon atmosphere.After completion, the Lil was removed by refluxing in acetonitrile for 8hours. FIG. 4 shows an XRD trace of the product obtained. The verticallines 1 represent the standard trace for pure Li₂FeS₂ taken from theJCPDS database. The main peaks are co-incident with and have similarrelative intensities to these lines 1, indicating that the dominantproduct phase obtained was Li₂FeS₂. The remaining peaks correspond tosmall amounts of unreacted starting materials.

The product obtained was used to manufacture a cathode as describedabove. FIG. 5 illustrates three cycling curves which indicate that thecathode could be repeatedly charged and discharged. This demonstratesthat the product was suitable for use as a cathode material for alithium rechargeable battery.

EXAMPLE 4

Li₂S and FeS₂ were reacted together in a molten salt solvent of lithiumchloride, LiCl, at 700° C. for ca. 2 hours, under an argon atmosphere.After completion the lithium chloride was removed by refluxing inpyridine for 8 hours. FIG. 6 shows an XRD trace of the product obtained.The main peaks are coincident with the lithium iron sulphides, Li₃Fe₂S₄,Li₂FeS₂ and Li_(2.33)Fe_(0.67)S₂. Unlike the other examples, a singlepure product was not obtained. These products are known to be suitableas battery cathode materials (A. G. Ritchie and P. G. Bowles, Processfor Producing a Lithium Transition Metal Sulphide, WO 00/78673 A1, 28Dec. 2000).

The examples described above demonstrate that the process of the presentinvention is suitable for use in the production of Li₂FeS₂ and that theproduct so obtained can be used as a cathode material in the manufactureof lithium rechargeable batteries. The process is significantly quickerand requires considerably less energy input than the conventional solidstate synthesis of lithium iron sulphide. This leads to significantreductions in the cost of the material.

Although described with reference to the production of lithium ironsulphide, it will be clear that the process could equally be used toproduce other lithium transition metal sulphides.

1. A process for the synthesis of lithium iron sulphide Li₂FeS₂, inwhich reactants consisting of lithium sulphide Li₂S and iron sulphideFeS react, under an inert atmosphere, in a solvent consistingessentially of a molten lithium halide salt, or a mixture of moltenlithium halide salts, so as to produce Li₂FeS₂ as the dominant productphase, and recovering the product by dissolution of the molten salt ormixture of molten salts in at least one organic liquid.
 2. The synthesisprocess according to claim 1 wherein the molten lithium halide salt ormixture of molten lithium halide salts comprises at least one of lithiumfluoride, lithium chloride, lithium bromide or lithium iodide.
 3. Theprocess for manufacturing a cathode comprising producing lithium ironsulphide Li₂FeS₂ by a process according to claim 1, and subsequentlyusing the sulphide so produced to manufacture the cathode.